The present invention relates to a femoral prosthesis for use in hip joint replacement procedures. More specifically the invention relates to a novel modular prosthetic head for a femoral prosthesis.
In one embodiment, the present invention includes as components a hollow ball component having a polymeric filler with a molded tapered socket for connecting the ball component to a prosthetic femoral component stem having a conically tapered trunion.
Artificial joint prostheses are widely used to restore joint mobility to patients affected by a variety of conditions, including degeneration of the joint and bone structure. Typically, the degenerated bone structure is replaced with an orthopedic implant that mimics, as closely as possible, the structure of the natural bone and performs its functions. The satisfactory performance of these implants can be affected not only by the design of the component itself, but also by the surgical positioning of the implanted component and the long-term fixation of the implant. Improper placement or positioning of the implant can adversely affect the goal of satisfactorily restoring the clinical bio-mechanics of the joint as well as impairing adequate fixation of the component when implanted.
Orthopedic implants are constructed from materials that are stable in biological environments and withstand physical stress with minimal or controlled deformation. Such materials must possess strength, resistance to corrosion, biocompatibility, and good wear properties. Also, the implants include various interacting parts, which undergo repeated long-term physical stress inside the body.
For these reasons, among others, the bone/implant interface and the connection between various parts of the implant must be durable and resistant to breakdown. This is especially important since installation of an orthopedic implant often involves an extensive medical procedure, and therefore replacement or revision of the installed implant is typically even more difficult. The requirements for the useful life of the implant continue to grow with the increase in human life expectancy and use in younger patients. As implants improve, more younger patients are considered as implant candidates. It is therefore desirable to develop implants that, while durable in their own right, minimize the difficulty of replacement or revision surgery should the implant eventually fail.
The strength and longevity of implants in large part depend on the bone/implant interface. Various methods of connection are known in the art. For example, a hip joint is a ball-in-socket joint, and includes a rounded femoral head and a cup-like socket (acetabular cup) located in the pelvis. The surfaces of the rounded femoral head and the acetabula cup continually abrade each other as a person walks. The abrasion, along with normal loading, creates stress on the hip joint and adjacent bones. If the natural femoral head or the acetabular cup is replaced with an implant, this stress must be well tolerated by the implant's bearing surfaces to prevent implant failure.
One type of hip prosthesis is a resurfacing femoral head. In this design, the natural bone neck is preserved and a hollow metal shell is implanted on the prepared head area of the femur. The inside or distal surface is covered by cancellous bone. The metal shell replaces diseased natural cartilage. Because the acetabular cartilage will wear out over time, this resurfacing is considered temporary solution and total hip replacement is eventually conducted. Hollow metal shell designs are well-known (UK 719,308, US 2010/0121458 A1). The shell is typically made of CoCr alloy. Especially, UK 719308 discloses a resurfacing prosthesis that is composed of CoCr shell filled with plastic core, where the plastic core directly contacts cancellous bone.
The total hip replacement replaces diseased cartilage with both a prosthetic femoral head and acetabular cup. One design of the femoral head is a one piece component integral with a femoral stem. In this design, the neck length is fixed, however a patient's femoral neck length may vary from person to person. Another femoral prostheses comprises a separate stem part and a head part. The neck length is easily adjusted by different offsets to fit different patient neck lengths. The modular design is easy to use, and has become popular over the past 20 years. The stem part comprises a body section for location in the intramedullary cavity of the patient's femur, and a neck section at its proximal end for carrying the modular head part. The head part comprises an outer bearing surface formed as part of a sphere, which is for articulation in a cup component implanted into the patient's acetabulum. The stem part and the head part usually are separate components. The proximal end of the neck section of the stem part comprises a conically tapered trunion and a distal side of the head part comprises a tapered bore adapted to receive the trunion. This configuration allows for the femoral stem to be more readily implanted into the femur initially, and for a particular head part to then be chosen to suit during the procedure. Multiple femoral prosthetic head parts may be provided with, for example, different offsets and diameters for mating with different diameter acetabular cups. Modular prosthetic heads are typically made of solid metal ground into a spherical shape and include a tapered socket, such as a Morse taper in a distally facing surface thereof. However, the traditional metal-on-metal taper junction may have corrosion and fretting problems, especially with a titanium stem against a CoCr head.
One solution for the taper corrosion is to change the metal head into ceramic head. Because ceramic is non-electric conducting material, there is no galvanic corrosion problem. However, ceramic heads are brittle compared to metal heads.